A radiotherapy treatment system typically includes a gantry that positions a radiation delivery apparatus, such as a linear accelerator (“linac”), around a patient during radiotherapy. A linac may include an electron gun with an electron source that emits electrons by thermionic emission. The electron source may be a cathode located in a vacuum tube. A directly-heated cathode may be referred to as a “filament” and an indirectly-heated cathode may be referred to as a “cathode heater.”
FIG. 1 shows a conventional circuit 100 for powering an electron source. A variable transformer 102 controls the voltage applied to the primary side of a filament transformer 104. Filament transformer 104 has the appropriate voltage ratio to change the output of variable transformer 102 to the voltage required for an electron source 106, such as a filament, to emit electrons toward a grounded anode 107. A pulse transformer 108 has a primary winding 110 coupled to a pulse generator 112, and a pair of unity-coupled secondary (or “heater”) windings 114 and 116 that feed the high-voltage to filament 106. The difference in the voltages at the “right” two terminals of unity-coupled secondary windings 114 and 116 is equal the difference at the “left” two terminals of the unity-coupled secondary windings 114 and 116 in FIG. 1 (e.g., 6 volts at 60 hertz). However, the mean voltage at the two terminals on the right will include a pulse waveform coupled from the pulse voltage in primary winding 110 (e.g., tens of kilovolts pulse amplitude and negative polarity.)
Use of the same reference numbers in different figures indicates similar or identical elements.